The present invention relates to gas and liquid fueled turbines and, more particularly, to methods of operating combustors having multiple nozzles for use in a turbine wherein the nozzles are staged between different modes of operation, and to the compact configuration that may be realized therewith.
Dry Low NOx technology is routinely applied for emissions control with gaseous fuel combustion in industrial gas turbines with can-annular combustion systems through utilization of premixing of fuel and air. The primary benefit of premixing is to provide a uniform rate of combustion resulting in relatively constant reaction zone temperatures. Through careful air management, these temperatures can be optimized to produce very low emissions of oxides of nitrogen (NOx), carbon monoxide (CO), and unburned hydrocarbons (UHC). Modulation of a center premix fuel nozzle can expand the range of operation by allowing the fuel-air ratio and corresponding reaction rates of the outer nozzles to remain relatively constant while varying the fuel input into the machine. Detailed methods for controlling or operating such a machine on natural gas are described for example in Davis, Dry Low NOx Combustion Systems For GE Heavy-Duty Gas Turbines, GER-3568F, 1996 and in U.S. Pat. Nos. 5,722,230 and 5,729,968, the disclosures of which are incorporated herein by this reference.
Liquid fuel is commonly supplied in industrial gas turbines with diluent injection for emissions control from approximately 50 to 100 percent of rated load. Water or steam is generally used as the diluent. Combustors with capability of operating on either gaseous or liquid fuels are well established and examples thereof are described in the aforementioned publications.
The problems associated with dual fuel machines include the packaging requirements associated with locating a number of fluid passages within a limited volume and the development of an effective methodology to control the operation of the machine while meeting the ever-lower emissions levels required by environmental agencies throughout the world. Solving these problems is of particular difficulty for small industrial gas turbines with can-annular combustion systems with lower than 35 Megawatts power output.
The nozzle configuration and control methodology of the invention is adapted to provide a compact means for configuring and operating an industrial gas turbine on either gaseous or liquid fuel while utilizing fuel staging to achieve very low emissions. More specifically, the invention is embodied in a configuration and operational methodology wherein the outer fuel nozzles are used for delivery of a portion of the premix gaseous fuel and all liquid fuel. Water injection for emissions control when operating on liquid fuel and atomizing air are also supplied entirely by the outer fuel nozzles. The central fuel nozzle is thus reserved for the supply of both premix gaseous fuel and diffusion gaseous fuel.
Thus, the invention is embodied in a gas turbine in which a plurality of combustors are provided, each having a plurality of outer fuel nozzles, e.g. from three to six, arranged about a longitudinal axis of the combustor, a center nozzle disposed substantially along the longitudinal axis, and a single combustion zone. Each outer fuel nozzle has at least one premix gas passage connected to at least one premix gas inlet and communicating with a plurality of radially extending premix fuel injectors disposed within a dedicated premix tube adapted to mix premix fuel and combustion air prior to entry into the single combustion zone located downstream of the premix tube. The center nozzle also has at least one premix gas passage connected to at least one premix gas inlet and communicating with a plurality of radially extending premix fuel injectors disposed within a dedicated premix tube adapted to mix premix fuel and combustion air prior to entry into the single combustion zone located downstream of the premix tube. The center nozzle further has a diffusion gas passage connected to a diffusion gas inlet. The diffusion gas passage terminates at a forwardmost discharge end of the center fuel nozzle downstream of the premix fuel injectors but within the dedicated premix tube.
The invention is further embodied in a method of operating a combustor wherein the combustor has a plurality of outer fuel nozzles in an annular array arranged about a center axis and a center nozzle located on the center axis, and wherein the annular array is selectively supplied with premix fuel, liquid fuel, water and atomizing air, and further wherein the center nozzle is selectively supplied with diffusion fuel and premix fuel, the method comprising the steps of:
a) at start-up, supplying the center fuel nozzle with diffusion fuel;
b) as the unit load is raised, supplying premix fuel to at least one of the outer nozzles in the annular array;
c) at part load, ceasing diffusion fuel flow to the center nozzle;
d) as load is further increased, initiating premix fuel supply to the center nozzle without adding to the supply of premix fuel to the outer fuel nozzles in the annular array; and then
e) supplying additional premix fuel to all of the outer fuel nozzles in the annular array and to the center nozzle as the turbine load increases.